This invention relates to a circuit sub-assembly, more particularly, to a circuit sub-assembly serving as a mounting for a superconducting electronic component.
Circuit sub-assemblies in particular applications are constructed as enclosures for packaging electronic components. Therein, a circuit sub-assembly will principally comprise an insulating substrate on which a circuit layer is formed, usually by a thick-film forming method such as the molybdenum-manganese (Mo-Mn) method.
The Mo-Mn method involves the preparation of a paste by mixing powders of such high melting-point metals as wolfram, molybdenum and manganese, with an organic binder and solvent. The paste is then screen-printed, or otherwise applied by a similar means, onto a green sheet for forming the insulating substrate. Next, the paste-printed green sheet is baked under a reducing atmosphere, and thereby a circuit sub-assembly is obtained, incorporating the insulating substrate and the circuit layer in one.
The insulating substrate composing the foregoing circuit sub-assembly is generally made from such oxidized ceramics as a sintered product of aluminum oxide or mullite, or a sintered product of non-oxidized ceramics such as aluminum nitride or silicon carbide, superficially covered by an oxide film.
The foregoing circuit sub-assembly is not suitable for the high-speed conduction of electronic signals, due to the high electrical resistivity of the material of which the circuit layer is composed. In particular, were the sub-assembly to thus be employed as a mounting for a superconducting component such as Josephson device, the realization of the innate function of the superconducting component, i.e., its high-speed driving function, would not be practicable.
Consequently, the use of niobium, which is a superconducting material, for the circuit layer is now under investigation. A circuit sub-assembly having a circuit layer composed of niobium is capable of conducting an electronic signal rapidly; and in such an application accordingly the high-speed function of a superconducting component is realized. The niobium film composing the circuit can be formed onto an insulating substrate by the thin-film method.
During the process of forming the circuit layer onto the insulating substrate, however, since niobium is easily oxidized, it tends to react with oxygen contained in the insulating substrate. The resulting oxides deprive the niobium of its superconductivity, again making impracticable the high-speed conduction of electronic signals, even wherein the circuit layer of the circuit sub-assembly is composed of niobium.